From the Spanning Sync blog: "Google Calendar is currently experiencing some technical problems. Internally we often refer to building an application on top of Google Calendar (which is itself still in beta) as 'building a house during an earthquake.'"
This makes me think of standards as seismic safety for software. With a standard, one can have a standard server running independent of the Internet that one can test against. When the server platform is a service and not a standard, if the service goes down, anything you're trying to build on top of it slams to a halt as well.